The design, synthesis, and photoelectrochemical characterization of Co 3 (PO 4 ) 2 , a hydrogen evolving catalyst modified with reduced graphene oxide (RGO), is reported. The 3 D flowerlike Co 3 (PO 4 ) 2 heterojunction system, consisting of 3 D flowerlike Co 3 (PO 4 ) 2 and RGO sheets, was synthesized by a one-pot in situ photoassisted method under visible-light irradiation, which was achieved without the addition of surfactant or a structure-directing reagent. For the first time, Co 3 (PO 4 ) 2 is demonstrated to act as a hydrogen evolving catalyst rather than being used as an oxygen evolving photoanode. In particular, 3 D flowerlike Co 3 (PO 4 ) 2 anchored to RGO nanosheets is shown to possess dramatically improved photocatalytic activity. This enhanced photoactivity is mainly due to the staggered type II heterojunction system, in which photoinduced electrons from 3 D flowerlike Co 3 (PO 4 ) 2 transfer to the RGO sheets and result in decreased charge recombination, as evidenced by photoluminescence spectroscopy. The band gap of Co 3 (PO 4 ) 2 was calculated to be 2.35 eV by the Kubelka-Munk method. Again, the Co 3 (PO 4 ) 2 semiconductor displays n-type behavior, as observed from Mott-Schottky measurements. These RGO-Co 3 (PO 4 ) 2 conjugates are active in the visible range of solar light for water splitting and textile dye degradation, and can be used towards the development of greener and cheaper photocatalysts by exploiting solar light., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)